1. Field of the Invention
The present invention relates to a capacitor microphone that has excellent directional frequency response up to a high frequency domain, and excellent sensitivity.
2. Description of the Related Art
Capacitor microphones have a basic configuration such as that disclosed in Japanese Patent Application Publication H2-237300. More specifically, capacitor microphones include: a capacitor microphone unit composed of a diaphragm a fixed pole that are provided facing each other via a spacer; and a sound signal output unit including an impedance converter that converts vibration of the diaphragm due to sound waves into an electrical signal as a change in capacitance.
A unidirectional capacitor microphone includes: a front acoustic terminal with which sound waves from a sound source are directly applied to the front surface side of a diaphragm; and a rear acoustic terminal with which the sound waves are applied to the rear surface side of the diaphragm.
A capacitor microphone unit is classified as small or large according to its bore diameter. Generally, the classification is such that, a capacitor microphone unit having a bore diameter equal to or smaller than 20 millimeters is classified as small, while a capacitor microphone unit having a bore diameter larger than 20 millimeters, e.g., 1 inch (25.4 millimeters), is classified as large.
In a small capacitor microphone unit with bore diameter equal to or smaller than 20 millimeters, the distance between a front acoustic terminal and a rear acoustic terminal can be made small. Thus, a small capacitor microphone has excellent directional frequency response in a high frequency domain. Unfortunately, the diaphragm therein has small area. Thus, a small capacitor microphone has poor sensitivity and S/N ratio.
On the other hand, a large capacitor microphone unit having bore diameter larger than 20 millimeters has a diaphragm of a large area. Thus, a large capacitor microphone has excellent sensitivity and S/N ratio. Unfortunately, the distance between a front acoustic terminal and a rear acoustic terminal is large and therefore, frequency response in high frequency range is poor.
S/N ratio depends on how the impedance converter is designed. Generally, better S/N ratio can be obtained with larger effective capacity.
A capacitor microphone can have higher sensitivity by, for example, increasing the driving forte for a diaphragm, lowering an impedance of a capacitor microphone unit, or increasing the area of a diaphragm (using a large capacitor microphone unit).
With a capacitor microphone unit having a diaphragm of a large area, higher sensitivity can be provided but frequency response in high frequency domain is degraded. A capacitor microphone is known that solves such problems and reduces intrinsic noise without degrading directional frequency response (see, for example, Japanese Patent Application 2006-5710).
In a capacitor microphone disclosed in Japanese Patent Application Publication 2006-5710, capacitor microphone units are connected in parallel. Therefore, sensitivity is difficult to be improved.